Optical catheters for performing in-vivo spectrophotometric measurements in the blood stream or elsewhere within living organisms are well-known in the art. (See U.S. Pat. No. 3,847,483) These have most commonly been used for the performance of oximetry, i.e., measuring the relative amount of the total hemoglobin within the blood stream that is in the oxygenated form. While prior art optical catheters can be used successfully for perfoming oximetry, they have a shortcoming which is of major importance to the medical practioner in the care of critically-ill patients. The catheters of the prior art require that an individual calibration be performed for each and every individual catheter that is to be used, in order to obtain accurate oxygen saturation measurements.
To perform this calibration, commonly a sterile optical catheter is inserted through the wall of a blood vessel of interest and advanced so that its tip is at a position within the flowing blood stream where it is desired that oxygen saturation measurements be made. The patient is frequently given a particular gas mixture to breathe; commonly a mixture enriched in oxygen or depleted of oxygen or two such mixtures sequentially, which causes the patient's blood to attain an oxygen saturation level in the regions of interest. Then, as blood samples are withdrawn (most commonly through an open lumen of the optical catheter) measurements are made of the relative light reflectances or transmissions at the catheter tip for the various optical wavelengths used by the catheter oximeter system.
The blood samples must then be taken to a separate instrument (for example, a transmissionspectrophotometer located in a central laboratory) where an independent measurement of the oxygen saturation of the one or more blood samples is made. The results of this independent measurement are then returned to the catheter oximeter at the patient's bedside, so that appropriate changes may be introduced into the catheter oximeter. These changes may include changes in bias levels and/or gains of various amplifiers in order to correct for the deviation between the initial oxygen saturation measurement made at the time of blood sampling and the oxygen saturation measurement independently determined by the separate instrument.
This requirement for individual calibration of catheters has obvious and important deficiencies. One such deficiency is the delay between the time of catheter placement and the time at which accurate measurements of oxygen saturation utilizing the optical catheter can be obtained. This delay deprives the physician of important information at a time when such information is often of the utmost importance in caring for the patient. For example, at the time of delivery of a newborn infant with severe respiratory distress because of prematurity, or severe Rh Hemolytic Disease or with other disorders, the resuscitation of these sick infants (who may weigh only two to three pounds) is frequently a precarious and difficult problem. This resuscitation must be instituted immediately upon birth and the various therapeutic manipulations completed within a very short time period. Unfortunately, the time required to perform calibration procedures on prior art optical catheters interferes with these catheters being used to furnish blood oxygen measurements during the course of resuscitation to guide the physician in the resuscitation procedure.
A second deficiency associated with calibrating the optical catheters of the prior art relates to the uncertainties associated with the resultant calibration. Changes in blood oxygen level occur continuously and often very rapidly, making it difficult to be certain that the blood sample and the oximeter readings are truly correlated. Further, during the process of blood sampling through the catheter tip, significant variations in flow profile of the red blood cells in the region where the optical measurements are being made may introduce errors into the optical measurements. In addition, the manipulations of the blood sample required to perform oxygen saturation measurement with an independent instrument can introduce errors in the calibration procedure.
It is therefore highly desirable to provide catheters which do not require individual calibration, so that each catheter of a whole population of catheters can be used to obtain blood oxygen measurement immediately upon introduction of the catheter into a blood vessel of interest.